Former Northern Territory youth detainee Dylan Voller is suing three media organisations over comments made by members of the public on Facebook in a landmark defamation case.

The NSW Supreme Court heard the first day of the case on Wednesday.

Mr Voller, whose mistreatment in detention sparked a royal commission in 2016, has brought action over a series of claims made about him in the comments section of 10 Facebook posts on the pages of the Sydney Morning Herald,The Australian, the Centralian Advocate, Sky News Australia, and The Bolt Report.

Dylan Voller testifies at the Royal Commission.

The news items contained in the posts are not the subject of the defamation claim. Mr Voller’s lawyers have asked the court to decide that the users of the pages are liable for the defamatory comments left on their posts.

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In three statements of claim, Mr Voller said the comments, on various Facebook posts published in 2016 and 2017, carried a series of false and defamatory imputations including that he attacked a Salvation Army officer who visited him in detention, left the man blind in one eye, and is a rapist.

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Mr Voller says the defendants chose to be proprietors of Facebook pages for their own commercial benefit; invited and encouraged readers and viewers to use their Facebook pages; and knew or should have known there was a "significant risk of defamatory observations" in the comments of each post.

He said the defendants had the power to remove or hide the comments, and could have chosen to monitor the comments but did not do so.

Mr Voller, who is seeking damages, interest and costs, says he has been subjected to "hatred, ridicule and contempt" and continues to suffer distress and damage to his reputation "by reason of the publication of the matters".

Social media managers from The Australian and Sky News gave evidence on Wednesday about their organisations' use of Facebook pages, including the use of profanity filters and how problem users are blocked.

More social media managers are expected to give evidence on Thursday.

The liability of Facebook page owners for comments left by others is an issue that has never been tested in Australia and could prove to be costly for media companies.

At present, only people who post defamatory comments on Facebook or who are told about the comments and don’t remove them from their pages are liable.

Mr Voller’s barrister Tom Molomby, SC, said in old cases anyone along the chain of publication of a book or newspaper could be liable for publication.

He said the question is whether the defendants in this case contributed to the dissemination of the material complained of in a way that makes them liable for publication.